Many mechanical systems contain moving parts not directly linked through mechanical means whose position, timing, or speed must be monitored and controlled with correction schemes for safe or efficient operation. A prime example is the operation of diesel engine fuel injectors. These injectors are usually controlled either hydraulically through rapid compression of fuel or electrically through operation of a fast moving solenoid valve. In both systems, the timing and speed of the actual injection of fuel into the combustion chamber greatly depends on the characteristics of the fuel being used. This is especially true of biodiesel fuels that contain various entrained organic materials and gases that make the fuel compressible and change its viscosity or other characteristics that affect valve speed or timing.
Mechanical systems such as internal combustion engines usually contain a significant number of these moving objects. For instance, there are usually multiples of 4, 6, 8 or more cylinders in diesel engines utilizing fuel injectors each containing a moving valve or other object that must be monitored for efficient or safe operation. Each injector requires a separate sensor. The wiring of these sensors to a remotely located engine monitoring and control system must be designed to accommodate extreme temperatures and vibrations and adds cost and weight to the system. A method of reducing the amount of wires should be employed when implementing these position sensors for maximum efficiency and minimum cost. One widely accepted method of reducing the wiring is to provide output signals in the form of changes in current drawn by the sensor that is directly proportional to the position of the object being monitored. This allows the sensor to operate requiring only two wires; one to deliver operating voltage and current to the sensor and another to provide a ground reference and to form a complete path for the current through the sensor. An example is a sensor that draws zero milliAmperes when the object is at rest and draws 5 milliAmperes when the object is closest to the sensor, with intermediate currents being drawn when the object is between these extremes of movement. These sensors operate by drawing their current through an external resistance inline with their connecting wires such that the resistance develops a dropped voltage level that is directly proportional to the current through the sensor. For instance, connecting a 20-Ohm resistor inline with the 5-milliAmpere sensor listed above results in a varying voltage drop of 0 to 100 milliVolts across this inline resistor. This voltage drop is monitored by external devices to convert the current information into voltage information for further processing.
Mechanical systems such as internal combustion engines also are designed so that the objects that must be monitored are known to be moving within specific limits or windows of timing such that at least some objects are moving at times that other objects are known to be at rest. For instance, the internal combustion engine fuel injectors operate in sequences equally timed in relation to the rotational position of the crankshaft. For instance, injector number one opens between 0 and 25 degrees of rotation, injector number two operates between 50 and 75 degrees, and the like. A method of further reducing the number of wires required for these systems can be employed by multiplexing or connecting all sensors to the same set of wires and a single inline resistor. Since each signal from each individual sensor is known to be occurring within a separate period or window of time, monitoring equipment that also monitors this timing information can know which sensor output is being sampled at any particular time. In the example for the internal combustion engine, a timing signal may be developed from a separate sensor delivering the rotational position of the crankshaft that is used to inform the injector position sensor monitoring system which injector should be operating at any specific rotational position of the crankshaft. This information is used to tag or otherwise mark the pulse train from the monitoring resistor to identify each individual sensor output.
Position sensors used to monitor these moving objects generate an electrical signal that is proportional to the distance between the moving object and a fixed position. An ideal output signal contains only this information; however, several unwanted electrical signals generally characterized as noise are also usually generated or otherwise transmitted along with the desired position signal. These noise signals are generally divided into either low frequency or into high frequency noise. Higher frequency noise is usually easily filtered out with a low pass filter since the frequency of these noise signals is higher than the frequency of the position signal because moving objects are constrained to velocities that generate signals in or just above the audio or ultrasonic range and because in a well designed sensor these high frequency noise levels are usually several magnitudes in power level below the desired output position signal.
Most position sensing transducers also generate low frequency noise in the form of a slowly drifting or static DC offset, or error signals that may be a significant portion of the total overall signal. An example of such transducers is a Hall cell where the signal generated is produced by a magnet. The signal from this transducer contains a large DC offset voltage generated by the magnet and a smaller AC signal generated as the target changes the magnetic flux density. Another example is a capacitive or inductive sensor where the slowly changing signal is caused by semiconductor device drift caused by temperature or other changes. This slowly changing or static error signal causes numerous problems in employing two-wire current output position sensors. The generation of any signal current through the sensor causes power to be dissipated inside the sensor. This adds to the temperature of the devices in the sensor, reducing the maximum ambient temperature that the sensor can operate at and reducing overall sensor reliability. The addition of a relatively static or DC current through the output sensing resistor connected to any number of these sensors increases the voltage dropped across the resistor. This leaves less power for the sensors or means that the applied voltage must be increased to generate the required operating voltage for the sensors. This power is wasted and also requires a higher power capability for resistors, by way of example. Also, increased current through the sensor wires means they also must be increased in diameter to accommodate the increased power lost through their series resistance. A further limitation on these type sensors is that especially upon power-up, the sensor should desirably not draw a large amount of current and should automatically calibrate itself so that no excessive current is drawn at any time during its operation. For instance, on vehicles utilizing storage batteries, the initial power-up of these sensors usually occurs at the same time that the battery is being used to crank the engine, reducing the amount of power available to power the sensors.